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April 28, 2016
Exam #3:
Graded exams on Tuesday!
Final Exam
Tuesday, May 10
th, 10:30 a.m.
Room: Votey 207
(tentative)Review Session:
Sunday, May 8
th, 4 pm, Kalkin 325
(tentative)Office Hours – Next week:
moved to Wednesday, May 4
th, 2:00-3:30 pm
Nuclear Magnetic
Resonance (NMR)
Spectroscopy
Chem 221
Instrumental Analysis
Spring 2016
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Overview
Based on the interaction of RF EMR (up to ~1000 MHz) with matter (in a magnetic field)
-EMR interactions with spin states of nuclei -RF EMR: much lower energy than optical EMR First demonstrated in 1946
First commercial instrument: 1956 We will be concerned with:
origin of RF EMR interactions
how these interactions are measured
how chemical information can be obtained from NMR measurements
Theory: Quantum Treatment
Energies of nuclear spin states are quantized:
Nuclear Spin Quantum Number (I) where I = 0, ½, 1, 1½, etc.
Three Groups of Nuclei: 1. I=0
-non-spinning nuclei, no magnetic moment, even # p+& no -examples: 12C, 16O
2. I=½
-spherical spinning charge with magnetic moment -examples: 13C, 1H
3. I>½
-non-spherical spinning charge with magnetic moment -examples: 2H, 14N
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More Quantum Numbers
All nuclear spin states are degenerate unless in
a uniform magnetic field
Where they split into 2I + 1 states
Defined by Magnetic Quantum Numbers (m):
m = I, I-1, I-2 . . . . -I
So, for I=0: m = 0 (only 1 state) NMR inactive for I=½: m = ±½ (2 states) NMR ACTIVE
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Energy States
For an I=½ system:
E
No MagField MagField
m = -½
m = +½
∆
E = 2µ
β
B
oParticle magnetic moment:
2.7927 nuclear magnetons (for 1H)
Nuclear Magneton:
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In General:
Selection Rule:
∆
m = ±1For any value of I:
h
ν
= (µ
β
B
o
)/I
•So, ν will vary with applied field strength (Bo) •Example: for 1H, ν = 60 MHz @ B
o= 14,092 Gauss
A Classical
Perspective
A“classical” view will help us understand the
measurement process.
Consider a spinning charged particle in a magnetic field: •Particle will precess at a characteristic frequency (Larmor Frequency)
ν
o:ν
o=
γ
B
o/2
π
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“Classical” view of
Absorption
Application of another magnetic field (B1) perpendicular
to Bo and at a frequency = νoresults in:
Absorption of applied EMR
Spin flip of particle to excited state
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Instrumental
Continuous Wave (CW) NMR
original instruments used:Electromagnets (14 - 23 kG; 60 - 100 MHz) Fixed frequency RF source
Swept (variable) magnetic field -measure absorption of applied RF
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Back to Boltzmann!
Using Boltzmann statistics, we can determine the relative populations of each of the two spin states:
N
2
/N
1
= e
-
∆
E/kT
•∆E is very small (relative to optical EMR) for RF EMR •As ∆E ↓, N2/N1 →1 (so, N2≈ N1)
•BUT: for absorbance, we want N1 >> N2
•If absorption rate > decay rate, not much absorbance can occur before N1 = N2 (saturation)
•When transition is saturated, NO MORE ABSORPTION!
Decay (relaxation) Processes
Two decay routes (non-radiative):
1. Spin-Lattice Relaxation (T1)
-also called: longitudinal relaxation
-due to interactions between nuclear spin states and magnetic micro environments in the sample -magnetic micro environments must be at the
Larmor Frequency of the absorbing nuclei in
order to couple
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Coupling Efficiency: T
1
T1is the excited state lifetime associated with spin-lattice relaxation (it is inversely proportional to the extent of spin-lattice relaxation)
Temperature effects
-at some temperature, the frequency of molecular motion matches the Larmor frequency for a nucleus and coupling efficiency is at a maximum (T1 is at a minimum) -any change in temperature will result in an increase in T1(decreased coupling efficiency)
Any (e.g., viscosity) changes in lattice mobility will have a similar effect
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More on T
1
Processes
Other lattice components that can reduce T
1:
unpaired electrons (from radicals and paramagnetic species, like O2)
nuclei with I ≥1
Efficient Spin-Lattice Relaxation results in:
Decreased likelihood of saturation Larger absorption signal (CW NMR) Other effects?
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Spin-Spin Relaxation: T
2
Energy transfer with other magnetic nucleiNuclei must be in close proximity
Very efficient coupling in solids (T2 ~ 10-4sec)
Has no effect on saturation Will cause line broadening:
∆ν= (2π∆t)-1 (according to Heisenberg)
so: linewidth ∝1/T2
(T2 ≈10-4sec → ∆ν ≈103Hz)
In solutions: (<1 sec)
T
2<
T
1 (1-10 sec) Controls linewidths (~1 Hz) Affects saturationHow can S/N be increased?
Increase B
o-increases ∆E, increasing population difference between spin states, so more nuclei can undergo transitions
-How? Superconducting Magnets
Multiplex Signal Measurement
-small signal makes measurement limited by detector noise, so a multiplex measurement method should improve S/N
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Pulsed FT-NMR
At fixed B
o, irradiate sample with a range
of RF EMR frequencies . . . How?
-by pulsing a fixed frequency (νo) RF source, a range (∆ν) of RF frequencies is generated. -the extent of the range is determined by the pulse width:
∆ν
= 1/4
τ
(according to Heisenberg) where:τ
is the pulse width (seconds)18
The Pulsed FT-NMR
Measurement
RF Excitation Pulse Free Induction Decay19
FT-NMR:
Obtaining a Spectrum
Obtain maximum number of excited nuclei during the RF pulse (saturation)
Measure RF Emission (signal generated by nuclei spin flips back to ground state) when RF source is off - FID
FID contains emission from all excited nuclei all at
their characteristic (Larmor) frequencies Use Fourier Transform to convert time domain FID to frequency domain spectrum